Vegetable oils phenolic compounds

The reduction of the stable 1,1 -diphenyl-2-picrylhydrazyl radical (DPPH) has been used to assess the efficiency of antioxidants in beverages (Larrauri et al, 1999 Porto et al, 2000), vegetable oils (Espin et al, 2000) and of pure phenolic compounds (Madsen et al, 2000), reaction [16.17] ... 

Natural tocopherol mixtures are used as antioxidants, usually at levels up to 500 ppm, along with ascorbyl pahnitate to extend the antioxidant activity. At higher levels (>1000 ppm), a-tocopherol is considered to act as a pro-oxidant. As vegetable oils contain tocols at 200-800 ppm, further additions show only a limited effect. The tocols are very sensitive to oxidation and are more stable in esterified form where the all-important hydroxyl group is not free. However such compounds do not show antioxidant activity until they have been hydrolyzed in vivo to the free phenolic form (11). 

The stability of olive oil compared with other vegetable oils is attributed to the high-to-low ratio of oleic to linoleic acid, and to the degradation of the chlorophylls to pheophytins (60). In addition, olive oil is also rich in antioxidative phenolic compounds such as hydroxytyrosol (66). 

STEROLS Phytosterols are mostly associated with unrefined vegetable oils and exist as derivatives of phenolic acids (e.g., ferulic acid). Several studies are available on antioxidant activity of sterols and their derivatives from sources such as corn fiber, oats, and rice. These compounds can be obtained from the unsa-ponifiable fraction that is removed during vegetable oil refining. Triterpene alcohols and hydrocarbons (131), or sterols (Figure 20) from oats (132, 133), rice (134, 135),... 

Low levels of a wide range of phenolic compounds have been reported to be present in all vegetable oils, which is very important for the oxidative stabihty of the polyunsaturated fatty acids of these oils. The mostly studied phenolic constituents are perhaps those of olive oil (Figure 8). The level of phenolic compounds in olive oil... 

The book covers food analysis for beneficial compounds, such as the determination of folate, vitamin content analysis, applications for avocado metabolite studies, virgin olive oil component analysis, lactose determination in milk, and analysis of minor components of cocoa and phenolic compounds in fruits and vegetables. With contributions by experts in interdisciplinary fields, this reference offers practical information for readers in research and development, production, and routing analysis of foods and food products. 

Figure 6. The peroxyl radical-scavenging activities of various vegetable oils (A) and flavonoid and phenolic compounds (B). Oils in markets. Adapted with permission from references 8 and 18, Copyrights 1999American Association for Cancer Research (A) and ACS (B),...

One of the most important phenols in olive oil is hydroxytyrosol (3,4-dihydroxy-phenylethanol). Several studies have reported its benefits for human health, such as antioxidant and antimicrobial properties and beneficial effects on the cardiovascular system. As a result, research has been carried out in the last few years in order to use this compound in enriched foods (vegetable oils, milk, or infant formulas). A SIA optosensor was developed for the fluorimetric determination of hydroxytyrosol in several foods. The use of a solid support in the flow cell increases the sensitivity and the selectivity of the system (Llorent-Martinez et al., 2013). 

Subsequent work by Zambiazi and Przybylski (1998) also showed that fatty acid composition could only explain half of the oxidative stability of vegetable oils including canola oil. The other half was attributed to the amount and composition of endogenous minor components which can shorten or extend the shelf-life of an oil. Such endogenous components were later discussed by Przybylski and Eskin (2006) and included tocopherols, mono- and diacylglycerols, free fatty acids, phospholipids, chlorophylls and derivatives, carotenoids, phytosterols, phenolic compounds and trace metals. In addition, the position that the fatty acid occupies in the triacylglyc-erol can also affect stability. For example, the location of linolenic and linoleic acids on the sn-2 position has been reported to cause faster oxidation and lower stability compared to the same fatty adds on ml- and sn-3 positions. In contrast oleic acid at the sn-2 position proved stabler compared to its location on sn-1 and sn-3 positions (Neffetal., 1994,1997). 

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